1. 'D-Day Has Come'

Presented bySophie RaworthBroadcaster

By June 1944, the tide of the Second World War in Europe was turning against Adolf Hitler. The Russians were making headway in the east. In the south, British and American forces had landed in Italy and were slowly moving north. But the most direct route to Germany, through occupied France, lay untouched.

Invading France was no simple task. The Germans were expecting an attack. The north French coast was heavily fortified. Transporting thousands of troops across the English Channel and landing them on beaches in full view of enemy fire risked high casualties. To succeed would require intricate planning, ingenious invention and the mounting of the largest sea and airborne invasion the world had ever seen.

2. A 20th Century Armada

3. Planning every last detail

The Allied commanders faced a formidable task. The lives of hundreds of thousands of men, and the freedom of millions across Europe, would depend on them getting it right.

Eye in the sky

In the months before the invasion, Allied reconnaissance aircraft flew thousands of hours over northern France. On these daring and often low-level flights, they were unarmed apart from the cameras capturing enemy positions, defences and industry. Within hours, the images would be in the hands of 1,700 photo interpreters who built an incredibly detailed picture of the Allies' D-Day destinations.

'Send us your photos'

As early as 1942 the BBC was asked to issue a public appeal for postcards and photographs of the coast of Europe, from Norway to the Pyrenees. Millions were sent in. Combined with the aerial images, these holiday snaps helped the Allies choose their landing sites, and RAF model-makers used the data to create detailed 3D terrain models.

Beach survey

Small teams in midget submarines conducted covert surveys of the landing beaches to make sure they chose the optimum sites. They analysed water levels, beach gradients and German defensive obstacles and took sand samples to check the beaches could take the weight of Allied vehicles.

When to land

The Allies wanted to land at first light when it was low tide, so hidden obstacles could be seen. British mathematician Arthur Thomas Doodson was enlisted to work out the tidal patterns using mechanised calculators, revealing that 5-7 June would provide the best combination of full moon and ideal tidal conditions.

Dress rehearsals

Rehearsal invasions taught the Allies vital lessons for the real thing. At Slapton Sands in Devon, poor communications meant that men stormed the beach under a hail of live artillery fire, suffering many casualties. Standardised radio frequencies would be essential on D-Day itself.

Weather forecasting

A meteorological team, led by Captain James Stagg, pored over data from observing stations, reconnaissance flights and weather balloons, to work out what the weather conditions would be like in Normandy. Stagg predicted that 5 June would be too stormy for a landing, but a break in the weather would make the invasion possible the next day.

4. Innovation to overcome the enemy

Under the command of Field Marshal Erwin Rommel, Germany had prepared for the invasion its leaders knew would come. The Allies needed to think their way around the formidable firepower, obstacles, and minefields of the Nazi's ‘Atlantic Wall’.

HORSA GLIDER: The Allies needed to land troops silently behind enemy lines. This glider made from plywood and fabric was released by a plane over France and piloted to its target. Some reached their objective safely, but others crashed off course.

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HIGGINS BOATS: The Allies had to deliver large numbers of troops directly on to the beaches. This craft was designed to skim on the sea’s surface, with a forward facing ramp to unload soldiers. General Eisenhower said they helped win the war.

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THE 'DUCK': The Allies also needed to reinforce and resupply their troops across the beaches. This six-wheeled amphibious truck was effective at carrying equipment and supplies over water and land. Some sank due to high-seas and being overloaded.

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SWIMMING SHERMAN: The Allies needed heavy fire-power to overcome German defences. This tank swam to shore using a flotation screen and propellers, before rolling up the beach and opening fire. Some were released early and sank, drowning the crews.

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CRAB TANK: The exits to the Normandy beaches were littered with mines. This tank had a fitted flail rotor to clear them and discs to cut barbed wire. Although it moved slowly it was effective at its task.

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CROCODILE: Beyond the beaches the Allies needed a weapon that would make the enemy retreat or surrender. This tank fitted with a flame thrower was much feared. Although limited in range, many enemy bunkers surrendered as soon as it was fired.

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5. Building a DIY harbour

Delivering an invasion force of the scale of D-Day would require a huge harbour. Capturing one would be far too dangerous; instead the Allies brought their own.

Designed by Major Allan Beckett of the Royal Engineers, the temporary 'Mulberry' harbours were built over six months by around 55,000 workers from 210,000 tons of steel, 1,000,000 tons of concrete. They still stand as one of the greatest civil engineering feats of modern times.

6. The art of deception

Topfoto and Getty Images

From top: The Atlantic wall, spy documents and a fake tank

The Allies knew that even with their combined strength of forces they would be up against formidable German defences. Surprise, would be vital. So they created a vast deception to mislead the Germans about when and where the invasion would take place.

Bodyguard

The plan was known as Operation Bodyguard, after Winston Churchill’s comment that in wartime, the truth should “always be attended by a bodyguard of lies.”

Double cross

The Allies used double-agents to feed information back to their German controllers. Elaborate tricks suggested the Allies might invade across the narrowest part of the English Channel, at the Pas de Calais, or through Gibraltar, and even Norway. Three agents, known as ‘Garbo’, ‘Brutus’ and ‘Tricycle’ were particularly valuable as they were well-trusted by their German controllers.

Artificial army

Backing up the spies' stories were fake armies. The most important of these was the hoax 1st U.S Army Group in Kent, which was placed under the command of U.S General George Patton.

Inflatable tanks

Roads and fields were lined with dummy tanks, trucks and aircraft. Fake landing craft were placed along the southeast coast of England. German Luftwaffe planes were occasionally allowed to fly over to take aerial photographs. The Allies also simulated the radio communications that would be expected for such a group.

Pretend paratroopers

Nearer the time of the invasion the Allies dropped twice as many bombs on the Pas de Calais as they did on Normandy. Even on D-Day itself, Allied planes dropped dummy paratroopers and tin foil to confuse German radar and maintain the idea the invasion would come further north.

Keeping the secret

The real invasion plans were kept top secret, with only the most senior British and American commanders in the know. Movement in the south of England was restricted in the months leading up to D-Day, with a coastal strip up to 50 miles deep in places out of bounds to all but authorised personnel.

Intercepted German transmissions showed that when it came, the Germans were still thoroughly confused as to the time and place of the actual invasion.

7. Were there alternatives to D-Day?

The D-Day invasion was incredibly risky. Casualties were high, but could have been worse. How else could the Allies have brought the war to an end?